Clydette M. Alsup
Department of Agriculture, Missouri State University

Answers to Questions:

Q.  1. Which plant extract inhibited the growth of galls? Do these substances have any known similarities? If inhibition occurred, was it slightly effective, effective, or highly effective? If inhibition was slightly effective, how might the effectiveness be improved?
Similarities of effective substances may include factors such as plant genera or families, or plant parts screened. The degree of inhibition depends on observations. If inhibition was slightly effective, it might be improved by increasing the concentration of the plant extract, or by treating the substrate (carrot disk) with the extract for a longer time interval. Combinations of materials might also improve effectiveness.

2. Which of the plant materials or other substances had no effect on the growth of galls? Do these substances have any known similarities?
Materials that had no effect on growth of the galls depend on student observations.

3. Why was it important to use sterile techniques in this investigation?
It is important to use sterile techniques in this investigation to prevent contamination by pathogens and saprophytes that might be present on students' hands or in the lab environment.

4. What variable factors could have affected the effectiveness of the screened substances in this investigation?
Variables that could have affected the effectiveness of the screened materials include plant species, age and health of the plant, time of day plant materials were harvested, length of time between harvest and the beginning of the experiment, storage conditions prior to the beginning of the experiment, location on the plant where samples were obtained, how well plant material was pulverized, amount of distilled water added to plant material, amount of time the carrot disks were soaked in the plant extract, lighting, temperature, and use of sterile techniques.

5. Why do plants vary so much in their active ingredients? What are the adaptive features of these active ingredients?
Active ingredients (secondary metabolites) in plants vary with climate, types of predators they have evolved to resist, competition from surrounding plant species, and factors such as whether the plant disperses its seed by animal consumption. Active ingredients include phenols, alkaloids, saponins and other glycosides, and terpenes. These active ingredients allow plants to defend against pests (pathogens and insects) and predators.

6. How might the knowledge gained from this lab exercise be used in agriculture to control A. tumefaciens?
Plant materials that appear to inhibit growth of A. tumefaciens would be analyzed for their active ingredients. Each ingredient could be tested alone or in combination with other ingredients for effectiveness as a bactericide. Bactericides could then be manufactured from plant compounds or synthesized from the ingredients that actively inhibit the bacteria. In addition, agricultural scientists might be able to genetically engineer plants capable of producing their own substances to inhibit pathogenic bacteria.